Acoustic tools are useful in providing a large range of information regarding formation and borehole parameters adjacent the tools. A primary use of acoustic borehole measurements is the estimation of compressional and/or shear wave formation slowness. The understanding of a recorded waveform in a borehole is quite simple. However, usually the effect of reflected waves from surface boundaries and noise corrupt the recorded data and require an interpretation process. Nevertheless, there have been various methods for interpreting acoustic data, with varying degrees of success in an open hole environment.
In a cased hole environment, however, the complexity of recorded waveforms is much more difficult to interpret. Yet in many difficult well conditions, casings are required. Recent advances in geophysics call for velocities (or slowness measurements) all the way to the surface of a well bore, while in many instances shallow formations can only be logged after casing is set. Thus a method to measure slowness in cased boreholes is desired.
Obtaining compressional and/or shear slowness measurements through a casing is challenging because the effect of the casing depends on the different bonding conditions of the casing to the formation and can therefore mask the formation slowness. Typically the casing signal corrupts the recorded waveforms and induces difficulties in the processing of data to obtain useful slowness measurements. Adding to the difficulty is the challenge of predicting the form and duration of the casing signal as the casing signal is dependant both on the formation slowness and the bonding conditions. The casing may induce a poor coherence for the slowness measurement log, and sometimes results in the loss of the entire formation signal.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems outlined above.